The present invention is directed to a method of converting an oxygenate feed to an olefin product using a silicoaluminophosphate molecular sieve catalyst.
Ethylene is an important petrochemical. In 1998 about 80 million tons of ethylene were produced, and demand is expected to reach 100 million tons by 2003. The primary use for ethylene is as a monomer for the production of low and high density polyethylene. Approximately 60% of world ethylene consumption goes into making polyethylene for such products as plastic films, containers, and coatings. Other uses include the production of vinyl chloride, ethylene oxide, ethylbenzene and alcohols. Presently, about 90% of the ethylene is produced by the steam cracking of light paraffin, naptha, and gas oil.
Propylene is another important raw material. In 1998 about 46 million tons of propylene were produced, and demand is expected to reach 60 million tons by 2003. About 55% of the world consumption is directed to the production of polypropylene. Other important end products include acrylonitrile for acrylic and nylon fibers, and propylene oxide for polyurethane foams. About two-thirds of the propylene is produced from steam cracking petroleum feedstock, and the remaining third as a by-product of FCC gasoline refining.
A potential alternative to producing ethylene and propylene from petroleum feedstock is to use an oxygenate feedstock. A particularly promising oxygenate feedstock is methanol. Methanol is readily produced from synthesis gas, which is derived from the reforming of natural gas. Large scale production of methanol from xe2x80x9cstrandedxe2x80x9d natural gas may provide methanol at a price that would allow methanol to be economically competitive with petroleum feedstock for the production of ethylene and propylene.
One way of producing ethylene and propylene is by the catalytic conversion of methanol using a silicoaluminophosphate (SAPO) molecular sieve catalyst. For example, U.S. Pat. No. 4,499,327 to Kaiser, discloses making olefins from methanol using any of a variety of SAPO molecular sieve catalysts. The advantage of using SAPO based catalysts, particularly SAPO-34 based catalysts, is that such catalysts have relatively high ethylene and propylene selectivities. However, SAPO catalysts undergo relatively rapid deactivation due to coke formation.
If an oxygenate, such as methanol, is to be used as a source of ethylene and propylene, improved catalysts or process conditions are needed to increase the production of ethylene from a given amount of oxygenate. This invention describes a process to increase ethylene selectivity in an oxygenate to olefin conversion process.
The invention is directed to a process of increasing the ethylene selectivity of an olefin product in an oxygenate to olefin conversion process by using an oxygenate feed containing methanol and acetone. The method includes contacting a silicoaluminophosphate molecular sieve catalyst with an oxygenate, preferably methanol, to convert a portion of the oxygenate to a product containing olefin, the oxygenate feed comprising from about 1% to about 15% by weight, preferably from about 1% to about 8% by weight, more preferably from about 2% to about 5% by weight acetone, and separating the ethylene and propylene from the olefin product. The method may further include recovering a portion of unreacted acetone. Preferably, the unreacted acetone is recovered as a dilute acetone stream, which is then directed to the oxygenate feed or oxygenate conversion zone.
The invention is also directed to a process of increasing ethylene selectivity in the olefin product by contacting silicoaluminophosphate molecular sieve catalyst, particularly regenerated and fresh catalyst, with a pre-feed containing acetone in a contact zone. The pre-feed contacted catalyst is then directed to an oxygenate conversion zone. The method includes contacting regenerated, SAPO molecular sieve catalyst, and optionally fresh catalyst, with a pre-feed containing acetone in a contact zone, directing the acetone contacted catalyst and an oxygenate to an oxygenate conversion zone to produce an olefin product, and separating the ethylene and propylene from the olefin product. The method may further include recovering a portion of unreacted acetone. Preferably, the unreacted acetone is recovered as a dilute acetone stream, which is then directed to the contact zone or oxygenate conversion zone. Also, the method may include separating a portion of the hydrocarbon product exiting the contact zone from the acetone contacted catalyst.
The pre-feed may further comprise methanol, ethanol, propanol, ethylene, propylene, butenes, or a mixture thereof. The pre-feed contains from about 1% to about 99% by weight, more preferably from about 30% to about 70% by weight, acetone. The amount of pre-feed added to the regenerated catalyst, and optionally fresh catalyst, is from about 2% to about 60% by weight, preferably from about 2% to about 20% by weight, most preferably from about 4% to about 12% by weight, CH2 per weight of catalyst.
The present invention will be better understood by reference to the Detailed Description of the Invention when taken together with the attached drawings and appended claims